You would not have guessed from their menacing swagger that this was to be a defining moment in their lives. Yet within the next electrifying hour, their demeanour would become more sober as a stark choice was hammered home.

That choice was delivered by police officers, in front of a judge, at the first "gang call-in" ever held in England. It was this: change and we will help you change; carry on as you are and we will come after you. They would hear from former gang members and listen to the raw testimony of the mother of a murdered teenager.

To their credit, these 10 were the smart few in their crew of 50. They had been wise enough to accept a police invitation to attend this American-imported initiative, which was tried successfully in Glasgow in 2008, leading to a 50 per cent fall in violent crime among a 500-strong target group.

Four hours earlier, with timing that only heightened the urgency of the capital's gang problem, a teenager was stabbed after being chased into a barber's shop, yards from the court.

Yet while forensic science officers from Haringey cordoned off Lordship Lane to comb it for evidence, inside the court police were attempting something pre-emptive and proactive, never before tried in London.

The proceedings began bizarrely at 6.30pm, with some blaring Eminem-style rap mood music. A vigilant police officer removed the water decanter from the dock because "it could be used as a weapon".

Minutes later Judge Shaun Lyons entered and addressed the youths, mostly white and aged 13 to 19, telling them: "Listen very carefully to what you hear today. It may change your life."

First up was Chief Inspector Ian Kibblewhite of Enfield police. He had just begun his speech when he suddenly broke off. "Is it funny?" he shouted, glaring at a couple of sneering gang members. "You may think you belong to a big gang, you may be 50 people, even 100, but we have 32,000 in our gang. It's called the Metropolitan police."

He told them: "You have a straightforward decision. If you choose to change, everybody in this courtroom will help you. If you don't we will come after you. We know where you live, who your families are, where you go to school. And as the Stephen Lawrence case showed, we have long memories, 18 years long, and we will come after you until we get you."

Next came Frank Cross, former consultant surgeon at the Royal London Hospital, who showed gory photographs - including a man with a meat-cleaver embedded in his chest - to clinically illustrate the devastating impact of a knife on the human body.

But it was the brave testimony of a grieving mother that transfixed the court. Nicola Dyer's 16-year-old son Shakilus Townsend was stabbed to death by seven teenagers in a notorious south London honeytrap murder three years ago. The mother of five spoke of the devastation his death had wrought: "I still remember the comments at my son's funeral when people said, 'Oh, he's a soldier.' But there is nothing soldier-like about being run down like an animal in the street and stabbed and beaten to death.

"My younger ones, six and seven, have lots of questions about how their older brother died. His sister is so depressed she wants her life to end. Sometimes I can't sleep. The other day, I had a dream and my son was in trouble. My first thought when I woke was, 'He's okay, he's in his bed.' It took a bit of time to dawn: he hasn't slept in his bed for three years. He'll never sleep in his bed again."

Ms Dyer, 36, had to pause several times, breathing deeply to regain her composure and to deliver the final message she wanted the youths to hear. "You might think that because you carry a knife, you will be the one who kills, but you might be the one who is killed, and your family will go through the nightmare that we did."

Jermaine Jones-Lawler, 20, from east London, was one of three former gang members to give the boys a piece of his mind. But instead of addressing them from the centre of the court like the others, he stood inches from the bullet-proof glass wall of the dock, looked into their eyes and said: "I've sat where you sit, in cuffs. I was looking at seven years for GBH, robbery, carrying a knife. I stabbed people.

"A lot of my friends got killed. I know men in prison who cry themselves to sleep every night and are doing life for murder.

"You are lucky - I wish I had been given the chance you have today. I'm not going to lie and tell you leaving [the gang] is easy, cos it ain't. But if you don't, next time you hear these words you will be in prison, or dead, and we'll be leaning over your coffin and saying, 'What a waste.' First step to leave this bullsh*t life is to ask yourself: is your life worth more than a postcode?"

You could have heard a pin drop. A former member of the Hotbloods gang in Florida, Ashton Dacosta, 24, spoke. "I was making two-and-a-half grand a day selling class-A. If you messed with my gang, we kidnapped your mother, your little sister," he said. "One guy got shot and his brain landed on my lap. I saw 10 people die. Now I see all my dead friends every time I go to sleep. It's not about area codes. When people ask me where I'm from I say, 'My mum.' You're at a pivotal point. Make the change."

It was riveting stuff, a mix of carrot and stick. At the end, each of the youths was given a card with a number to call that would "change their life".

An Enfield council officer said: "Contact us and we will work with you, there are lots of groups waiting to help you." Judge Lyons sent them away with one thought. "When you leave tonight, you will be led through the cells and out onto the street, but next time you're in that dock, you may lose years of your life. You have heard. Now you have a choice to make. Go away and think about it."

So how had it gone? Were the police disappointed to have only got 10 attendees when in Strathclyde they got an average of 40 at each call-in? "No," said Enfield's borough commander, Chief Superintendent Simon Laurence. "The turnout was similar to the pilot in Scotland. We see this as the first step of several call-ins over coming months."

The Get Money Gang, he added, are one of two serious gangs in Enfield responsible for 30 to 40 per cent of violent crime in the borough.

Will it work? With 400 gangs in London, including 60 "high-harm" groups engaged in serious crime, borough commanders will be watching closely.

Meanwhile, in Enfield, the police are waiting by their phones, hoping the Get Money Gang make the right call.

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